In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, a transfer roller is used for transferring a toner image formed on a photoreceptor body onto a surface of a sheet such as a paper sheet or a plastic film.
The transfer roller generally includes a roller body formed from an electrically conductive rubber composition which is prepared by blending rubber, a crosslinking component for crosslinking the rubber, and a foaming agent thermally decomposable to generate gas to foam the rubber and is imparted with electrical conductivity. More specifically, the roller body is produced by forming a tubular body from the electrically conductive rubber composition and crosslinking and foaming the rubber of the tubular body with heating, and then the transfer roller is produced by using the roller body. Acrylonitrile butadiene rubber (NBR) is typically used as the rubber for the roller body.
In Patent Document 1, it is contemplated to use styrene butadiene rubber (SBR) instead of the NBR, to use the foaming agent as well as a foaming assisting agent functioning to reduce the decomposition temperature of the foaming agent to accelerate the decomposition of the foaming agent, and to properly adjust the proportions of the foaming agent and the foaming assisting agent.
With this arrangement, the SBR is lower in unit cost than the NBR, so that the production costs of the transfer roller can be reduced. The foaming reduces the amount of the material to be used, thereby further reducing the costs. Further, the foaming reduces the weight of the transfer roller, thereby reducing the transportation costs and the like.
In addition, it is possible to impart the roller body with flexibility suitable for the transfer roller and to reduce foam cell diameters to improve the smoothness of the outer peripheral surface of the roller body by adjusting the proportions of the foaming agent and the foaming assisting agent within predetermined ranges.
Since the decomposition temperature of the foaming agent is reduced by the blending of the foaming assisting agent, particles of the foaming agent are liable to be decomposed substantially simultaneously in the generally entire tubular body in a very short period from the start of the heating. Thus, expansion of foam cells resulting from the respective foaming agent particles during the foaming is suppressed by mutual expansion power of adjacent foam cells. As a result, the foam cell diameters are reduced.
Therefore, the transfer roller including the aforementioned roller body ensures formation of higher-quality images with the smoothness of the outer peripheral surface of the roller body thus improved, and the cost reduction as described above.
In Patent Document 1, epichlorohydrin rubber for imparting the roller body with ion conductivity and ethylene propylene diene rubber (EPDM) for improving the ozone resistance of the roller body are used together with the SBR as the rubber for the roller body.